Indigestible NDF

Indigestible NDF (iNDF) is rapidly replacing lignin as the analysis of choice to measure the indigestible fiber in feedstuffs. While this small number may seem trivial, it is probably the single most important factor in calculating rates of digestion for NDF. Consider the fact that rates of digestion only apply to the potentially digestible NDF (NDF – iNDF = pdNDF). If we do not accurately measure the size of the potentially digestible pool, the rate is a bit irrelevant. It’s sort of like worrying about the interest rate on a bank account without considering how much money is actually in the account.

Traditionally, lignin has been used to estimate iNDF, but it has a number of drawbacks. Mainly, it does not have a consistent relationship to iNDF across feed types, it’s an imprecise chemical analysis, and the total range of lignin values in forages is narrow (about 10 units). In contrast, iNDF is a very uniform feed fraction, a more precise chemical analysis, and has a broad range of values (about 30 units). These factors all contribute to the fact that NIR calibrations for iNDF have about 1/3 of the error compared to lignin calibrations.

So, how does this help to balance better rations? First, iNDF will reduce the analytical error built into diets because both the chemistry and NIR measurements have less random error than lignin. Second, it will eliminate the artificial bias that exists in diet formulation because lignin does not have a uniform relationship to indigestible fiber across feedstuffs. Finally, iNDF will make it easier to differentiate high- and low-quality forages because it has more range.

iNDF can be measured reliably in a variety of In Vitro and In Situ systems, and the system largely determines how long it takes to reach a uniform extent of digestion. It’s important to work closely with your lab to ensure that the combination of system and time point is correct for your application of iNDF.